It is well-known that in order to conceal things and persons, in an area covered with snow, from inspection with the naked eye, they are covered with a white sheet material. Also, it is well-known that in order to provide the white cover sheet material, a conventional white pigment, for instance, titanium dioxide, may be used. However, the conventional white pigments have a property such that they absorb most of the incidental ultraviolet rays and hardly reflect the incidental ultraviolet rays, while snow reflects 70 to 90% of incidental ultraviolet rays. For this reason, when the conventional white sheet material placed on snow is scanned, by using an ultraviolet ray-sensitive inspecting means for instance, a special camera equipped with a filter permeable for ultraviolet rays or another special device, for example, a spectrophotometer, the conventional white cover sheet material is easily and clearly distinguished from the snow surface.
Accordingly, when the ultraviolet ray inspection is applied, the conventional white sheet cannot conceal materials or people placed on snow.